1. Field of the Invention
This invention relates to transparent sheets formed from blends of polycarbonate and polyethylene terephthalate resins, a process for their production and containers formed therefrom.
2. Discussion of the Prior Art
Polyethylene terephthalate (sometimes referred to as "PET") resins can be employed to prepare transparent film and sheet. Usually the resin is extruded into an amorphous flat sheet, which is then biaxially stretched and thereafter heat set to impart a desired degree of crystallization to the sheet. Such biaxially oriented and crystallized products are strong and clear but cannot readily be formed into containers since the process of biaxially stretching removes most of the extensibility of the sheet. If amorphous PET sheet is produced by rapid cooling of the molten sheet, a clear and transparent product may be obtained which is formable into containers. However, these containers soften at too low a temperature to permit their use in hot-filled food packaging applications where the filling may typically be at a temperature of about 150.degree. to 180.degree. F. or greater which facilitates rapid filling of viscous products as well as destroying bacteria. On the other hand, if PET sheet is produced by slow cooling of the molten resin, the product obtained is partially crystallized, milky and brittle and hence unsuited for container fabrication.
Although it is known that lower intrinsic viscosity PET resins may be modified by the addition of polycarbonates as is described in U.S. Pat. No. 3,218,372 to Okamura et al issued in 1965, in order to increase the hardness, strength and electrical properties of the molding material, such mixtures introduce additional problems. For example, polycarbonate resins employed herein are sensitive to decomposition at extrusion temperatures in the presence of other polymeric materials, such as PET, and tend to form bubbles of gas which are believed to be mainly carbon dioxide. The presence of these bubbles destroy the value of the sheet for thermally formed containers since holes develop and the optical properties are diminished. In addition, no prior process is known to the inventors which permits the extrusion of such blends into highly clear sheets having uniform transparency and low haze. This additional problem evidently arises from the wide dissimilarity of flow characteristics between the two resins of the Okamura et al. patent so that intimate mixing to obtain the very high degree of uniformity needed in transparent sheet is very difficult to achieve and in practice non-uniformities of various types such as localized surface roughness, flow streaks and other defects become readily evident. It would be desirable if such defects of PET and PET-polycarbonate blends were overcome to provide a practical process for extruding high clarity sheets which permits hot-filling to be used when clear containers are made from the sheet.
In our U.S. Pat. No. 3,956,229 (1976) there is described film and sheet formed from blends of 60 to 85 parts of PET having an intrinsic viscosity of at least about 0.90 and 40 to 15 parts of a polycarbonate resin. Such film or sheet, which has a degree of crystallinity in the range of about 20 to 40%, is essentially non-oriented and may be thermoformed into cook-in-trays and like articles. The film or sheet disclosed therein is formed by blending the polymers, extruding the blend at a temperature above about 500.degree. F. onto a moving support and cooling the support to a surface temperature of about 225.degree. to 380.degree. F. Although such film and sheet have requisite strength and toughness to be utilized for cook-in-tray applications, such sheets have a very high degree of haziness and consequently would not be suitable for applications wherein a clear sheet is desired. In U.S. Pat. No. 3,975,355 (1976) of two of the present inventors (Bollen and Amin), there are described film or sheet similar to that of our copending application but which also includes about 5 to 20 parts by weight of a non-acidic silica filler, such as novaculite. However, such film or sheet likewise has a degree of haziness which precludes its utilization in hot-filled applications, wherein a clear and transparent sheet is required.